Improving the Purity and Uniformity of Pd and Pt Nanocrystals by Decoupling Growth from Nucleation in a Flow Reactor

Despite significant progress in controlling the synthesis of noble-metal nanocrystals, the purity and uniformity of many products are still plagued by the unwanted nucleation events taking place in the growth stage. To this end, it is necessary to decouple growth from nucleation during a colloidal synthesis of nanocrystals. Taking Pd nanocubes as a typical example, here, we demonstrate that nucleation and growth can be separated from each other by re-designing the experimental setup while manipulating the reduction pathway of the precursor. Specifically, we pump the reaction solution through a tubular flow reactor and trigger a single burst of nucleation by subjecting the solution to an elevated temperature for a very short period of time to enable solution-phase reduction. The solution is then kept at room temperature for slow growth through surface reduction, producing pure and uniform Pd nanocubes. Due to the elimination of additional nucleation events during growth, the products exhibit high uniformity in terms of both size and shape. We elucidate the mechanistic details by quantitatively analyzing the reduction kinetics and monitoring the nanocrystals obtained at different stages of the synthesis. We further investigate the impacts of both temperature and duration of nucleation on the diversity of seeds and the quality of the resultant nanocrystals. This methodology has also been extended to the preparation of sub-5 nm Pt nanocubes and is potentially applicable to the synthesis of other types of colloidal nanocrystals.